The objectives of this study were to determine whether the maternal plasma corticotrophin-releasing hormone (CRH) concentration influences the amount of uterine contractility induced by infused oxytocin during induction of labour, and secondly to assess changes in CRH and beta-endorphin in response to stress during labour. Serial plasma CRH and beta-endorphin measurements were made in 40 women undergoing induction of labour and correlated with uterine contractility, cervical dilatation, length of labour, analgesic usage and fetal distress. The plasma CRH concentration did not change throughout labour. In subjects receiving infused oxytocin there was a significant positive correlation between plasma CRH and the amount of uterine activity, and a high plasma CRH level was associated with shorter labour. The plasma beta-endorphin level rose with progressive cervical dilatation and fell after epidural anaesthesia. The plasma CRH level did not correlate with the plasma beta-endorphin level or rise with fetal distress. We conclude that high levels of maternal plasma CRH are associated with an increase in the uterine contractile response to infused oxytocin. The maternal plasma CRH level does not vary in response to maternal or fetal stress, but beta-endorphin secretion does rise in response to the stress of labour and is influenced by pain perception.